Development of the Pectinihranchiata. 357 



from them, becomes thicker, and acquires a yellowish tint and 

 a cellular structure. Cells are also formed in the rotatory organs, 

 but these are neither so compact nor arranged so much upon one 

 another as in the foot. On the margins and on the surface of 

 the latter we observe very fine cilia. 



The rotatory organs are extremely clear and transparent. 

 Vogt* has best described their form, and, as our observations 

 agree with his, we shall not speak of them. We shall only re- 

 mark, that the cirrhi are shorter in Buccinum undatum than in 

 Actaon. The foot advances considerably in front, and we soon 

 observe the two auditory organs at its base (fig. 2^). These con- 

 sist of two round, clear vesicles, which are filled with a fluid as 

 limpid as water, and exhibit double outlines ; each vesicle is fur- 

 nished with a single otolithe. When the embryo is compressed 

 with a little force, the otolithes almost always break into four 

 regular fragments. The tremulous movement which most authors 

 have observed in the otolithes has not been remarked by us in 

 Buccinum undatum; and, although we have employed pretty 

 high magnifying powers, we have been unable to discover the 

 cilia on the inner wall of the vesicle. 



Whilst the rotatory organs and the foot are being developed 

 above, the membrane surrounding the lower part of the embryo 

 grows thicker, its outlines become strongly defined superiorly by 

 a distinct margin, and it thus forms the mantle (fig. 1 d, d, d). 

 This increases by a permanent exudation from the vitellus, and 

 cells are successively formed in the mass. On the bottom of 

 the mantle, a round, transparent, and membranous shell is then 

 formed (fig. 2 a). At the same time we observe, at the two 

 sides of the base of the foot, the two salivary glands, which are 

 pyriform (fig. 1), and in which round cells are formed by de- 

 grees. Their lowest portion, which is the largest, is filled in the 

 centre with a multitude of pigment grains, very strongly coloured. 

 Almost at the same time, we perceive the place of the mouth and 

 pharynx, and traces of the heart. The buccal orifice forms a 

 pretty large cleft, which is furnished with cilia ; it occurs at the 

 point where the two rotatory organs meet in front. It is con- 

 tinued into the pharynx, which has the form of a funnel, broad 

 above and narrow beneath ; all its inner portion is covered with 

 cilia. Grant t was the first who observed the heart in Buccinum 

 undatum, and called attention to its strong pulsation. He also 

 pointed out that the embryos of Purpura, Trochus, Nerita, Doris, 

 and AHolis, had at the sides of the head two round organs covered 

 with twisting cirrhi, by which movement was effected. He did 



* Annales des Sciences Naturelles, 3 serie, vi. p. 44. 

 \ Edinburgh Philosophical Journal, vii. p. 121. Unfortunately we only 

 know this Journal from the extracts given by other authors, 



